A surface-mounting type semiconductor device (semiconductor package) in which a power MOSFET or other semiconductor element is sealed with a resin is known. In the semiconductor device of this type, the leads used are flat leads and a heat dissipation plate is often provided on the rear surface of the semiconductor device. When the semiconductor device is mounted on a substrate, the heat dissipation plate is soldered onto the substrate together with the leads. Soldering both of these elements allows space saving to be achieved and also high heat dissipation properties to be obtained. In such a configuration, when a heat cycling is applied over a long period of time, a large stress is generated due to differences of the thermal expansion between the semiconductor device and the substrate. Therefore, there is a concern with the occurrence of solder cracks.